1. Field of the Invention
The present invention relates generally to earth boring roller cone rock bits.
More particularly, this invention relates to roller cone drill bits that leave uncut a significant size core of the formation being drilled to substantially enhance the drilling rate of the bit. The diameter of the core being left is preferably maintained by the use of inserts in the cone apices made of significantly harder and more wear resistant material than are the main cutting elements on the roller cones.
2. Description of the Prior Art
Roller cone rock bits are used extensively in drilling for petroleum, minerals and geothermal energy. Roller cone bits generally comprise a main bit body which can be attached to a rotary drill string. The bit body normally includes two or three legs which extend downward. Each leg has a journal beating extending at a downward and inward angle. A roller cone with tungsten carbide inserts (TCI bit), steel teeth (milled tooth bit) or other cutter elements positioned on its outer surface, is rotatably mounted on each journal or roller bearing. During drilling, the rotation of the drill string produces rotation of each roller cone about its bearing thereby causing the cutter elements to engage and disintegrate the rock.
Because of their aggressive cutting action relatively fast drilling rates are achieved, but very often roller bits designed to drill the entire hole bottom do not drill at acceptable rates.
U.S. Pat. No. 2,901,223 describes a "milled-tooth" roller cone drill bit with large steel teeth designed to leave uncut a substantial formation core at the center of the borehole bottom when drilling very soft, deformable strata to enhance the rate of penetration. The core diameter is maintained by the steel teeth at the apex or "spear-point" of each roller cone. The core is broken up when contacting a centrally and vertically positioned core breaker extending from the bit body. The broken core fragments are flushed out from under the bit by drilling fluid exiting from jet nozzles directed between the cones. This bit design did not gain much acceptance as all of the steel teeth wore away fairly rapidly. Although the wear of the outer teeth on the cones was not severe enough to stop the drilling process, the wear of the teeth on the cone apices allowed a core of ever-increasing diameter to form. This design also lacked intermeshing teeth that decreased the bearing size and prevented self-cleaning of teeth that can lead to bit balling. Neither the mechanical action of the core breaker mechanism, nor the jet hydraulic energy were sufficient to the break and flush the enlarged core from under the cutting structure of the bit, thereby stopping the drilling process prematurely. A costly "round trip" of the drill string was then necessary to replace the worn bit.
U.S. Pat. No. 3,134,447 shows a tungsten carbide insert type roller bit for drilling fairly hard and abrasive rock formations. This bit type was deliberately designed to leave a very small uncut formation core to alleviate the undercutting of the steel at the apex of the cone with subsequent loss of the carbide inserts. By terminating the cone before it reaches the bit center, a blunter cone can be made that will accommodate many more carbide inserts, thereby protecting the steel of the cone apex from abrading away and ultimately losing inserts. This bit, while useful in drilling a very limited range of very hard and abrasive rocks, does not compete in drilling the major range of sedimentary formations because of very slow drilling rates. The slow drilling rate is directly attributable to the massive concentration of carbide inserts in the apex areas of the roller cones, which produces a very low point loading of the rock by the inserts. This low point loading is generally insufficient to reach the fracture threshold of the rock being drilled.
A new roller cutter drill bit is disclosed which overcomes the inadequacies of the prior art.